Conventional permanent-magnet electrodynamic loudspeakers employ a diaphragm which is vibrated by an electromechanical drive. The drive generally comprises a magnet and a voice coil through which an electrical signal is passed. The interaction between the current passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to oscillate in accordance with the electrical signal, and drive the diaphragm to produce sound.
The coils or windings used are conductive and carry alternating current. In operation, the resistance of the conductive material causes the production of heat in the voice coil or winding. The tolerance of the driver to heat is generally determined by the melting points of the various components and the heat capacity of the adhesive used to construct the voice coil. As the DC resistance of the voice coil comprises a major portion of a driver's impedance, most of the input power is converted into heat rather than sound. Ultimate power handling capacity of a driver hence is strictly limited by the ability of the device to tolerate heat.
The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the driver increases, the DC resistance of copper or aluminum conductors or wires used in the driver also increases. For example, a copper wire voice coil that has a resistance of six ohms at room temperature has a resistance of twelve ohms at 270.degree. C. At higher temperatures, power input is converted mostly into additional heat rather than sound, thereby posing a serious limitation on driver efficiency.
It is therefore desirable to cool the voice coil under operation to maximize driver efficiency.
Previously it has been suggested to cool the voice coil by forcing air into the center of the magnet structure and over the coil windings. For example, U.S. Pat. No. 4,757,547 discloses an external blower which forces air over the voice coils to cool them. However, in practice this system has drawbacks. As the gap between the voice coil and the pole piece of the magnet is very small (approximately 0.010 inches) cooling can only be achieved by forcing air through this air gap at a very high air pressure. Under a high air pressure, the dome will take on a positive set and cause the coil to be no longer centered in the gap. This offset will cause second-harmonic distortion. Additionally, the blower can be loud and obviously non-musical, resulting in speaker distortion and excessive noise.
There have also been attempts to use the movement of the dome to force air past the voice coil through movement of the cone with a sealed magnet structure. This system also has its drawbacks in that the air gap between the voice coil and the magnet is too small to allow proper flow past the windings of the voice coil. While a higher power handling may be achieved with this structure, the sound quality is affected due to the air flow through the gap which causes changes in the motion of the dome or cone, resulting in distortion and a damped bass response.